Blunt force trauma, probably from ship strikes, was responsible for the deaths of five endangered North Atlantic right whales in the Gulf of St. Lawrence this summer, experts have confirmed. A sixth whale died from "chronic entanglement" in fishing gear.

The results of necropsies performed on the whales' carcasses were presented Thursday at the Atlantic Veterinary College in Charlottetown.

A total of 12 North Atlantic right whales have died in Canadian waters since June — an unprecedented number and a major blow for the critically endangered species, whose estimated global population has now dropped below 500 individuals.

Although the species is protected under the federal Species At Risk Act, it faces a number of human-caused threats throughout its range, the most common ones being entanglement in fishing gear and vessel strikes.

Right whales are a migratory species and travel regularly along the east coast of North America between Florida and Newfoundland. Little is known about their habits and distribution in the Gulf of St. Lawrence, but according to the necropsy report, published by the Canadian Wildlife Health Cooperative, researchers believe the whales have been frequenting the Gulf in greater numbers since 2015.

In August, in response to the deaths, Fisheries and Oceans Canada closed a snow crab fishery between the New Brunswick coast and the Magdalen Islands, imposed speed limits in the Laurentian channel, and stepped up surveillance. The whales typically migrate out of Canadian waters by December, and the ministry plans to spend the winter months consulting with the fishing and marine transportation industries, whale experts and scientists, Indigenous communities, and the U.S.-based National Oceanic and Atmospheric Administration (NOAA) on how to prevent another deadly summer for the species.

“We will continue to monitor the migration of the whales and the efficiency of our measures in order to act accordingly, based on evidence," Fisheries Minister Dominic LeBlanc said in a statement. "We are committed to doing what is necessary to help keep our right whales from harm, and we are considering all options in order to protect this iconic species."